1. Field of the Invention
The present invention relates to a color indication system, and more specifically, to a method of determining a reference value for the intensity associated with a color indication variable of a scanned color original.
2. Discussion of Related Art
If a white reference is to be determined as the reference value, it should be noted that the various support materials used for originals usually do not all have the same "white" color and therefore have different white reference values. On the one hand, this is due to the fact that although various kinds of white support materials, such as paper, which are available on the market are perceived as white, they nevertheless prove to have different white coloring if they are compared with one another. Secondly, this occurs in the case of originals which have been tinted (e.g., yellowed) due to sunlight or in some other way due to internal chemical action. Moreover, it occurs that a newspaper is printed on lightly tinted paper. When such originals are copies, reproduction of the background color is generally not required. In reproducing the background color, unnecessary toner material is used while no extra information value is added to the copy. Accordingly, it is a disadvantage when a user, feeding a yellowed original having only black and white information to a color reproduction system based on the U.C.R. principle having a black toner reproduction unit, finds that although the information is reproduced solely by the black toner unit, the color reproduction unit nevertheless has to come into operation solely and simply in order to reproduce the background color. This causes the cost price of a copy of such an original to increase quite significantly. Besides, in reproducing colored information it must be taken into account that pastel tints in a colored original may not be suppressed.
A method in which the maximum intensity values of color indication variables are determined for the original in its entirety, in fact during a pre-scan movement, and a white reference value (white point) for that original is calculated therefrom, is known from GB-A-1 541 578. EP-188 193 also describes a method in which determination of a white reference value of the entire original is effected by reference to a histogram compiled for each color indication variable. The methods described in these applications have, however, the disadvantage that they calculate only a single value for the white reference, which then has to be representative of the entire original. It is possible, however, that an original is not uniformly tinted, so that determining a uniform white reference value for the entire original is illogical. This should be noted as a disadvantage of the methods of determining a uniform white reference value as described in these applications. Moreover, the method described in EP-A-188 193 has the practical disadvantage that all the information relating to the color indication variables must first be stored in a memory and a then the white reference value of the original can be determined.
Similar considerations to those above also apply if another reference value, e.g. the black reference value, has to be determined. When black support materials are used, the materials again do not always have the same black background, and therefore have different black reference values, although these support materials are in fact perceived as being black. It is also possible that support materials for originals may have minor variations in a given color tint, e.g. as a result of consecutive manufacturing processes, and it may be desirable that the copies should have a uniform color tint as the background tint. In such a case, color reference values of the originals concerned should be determined, and they can then be converted to a uniform color reference value.